1. Field of the Invention
The present disclosure generally relates to a structure and method of making a heterojunction bipolar transistor (HBT) that offers high-speed operation by reducing a collector-base capacitance. An exemplary embodiment of the HBT may include a p-type intrinsic base comprising a boron-doped, silicon germanium (B+-doped Si1-x1Gex1) epitaxial crystal that is disposed on a top surface of an underlying crystalline Si layer, which is bounded by shallow trench isolators (STIs), and that forms angled facets on interfaces of the underlying crystalline Si layer with the shallow trench isolators (STIs), and a germanium-rich crystalline Si1-x2Gex2 layer epitaxial layer that is disposed on the angled facets. More particularly, a facet of the B+-doped Si1-xGex1 epitaxial crystal may affect boron out-diffusion from the epitaxial crystal. Yet more particularly, the germanium-rich crystalline Si1-x2Gex2 epitaxial layer that “wraps around” the p-type intrinsic base of the B+-doped Si1-x1Gex1 epitaxial crystal may act as a barrier to boron diffusion along an interface region with the angled facets from an extrinsic base layer into the crystalline intrinsic region of the HBT.
2. Description of Related Art
A heterojunction bipolar transistor (HBT) includes, for example, a silicon/silicon germanium (Si/SiGe) heterojunction that provides superior conduction for operation at high frequencies. The Si/SiGe heterojunction of the HBT is formed by epitaxially growing a crystalline SiGe layer on a crystalline Si substrate. Since the crystalline Si substrate and crystalline SiGe layer are made of materials that are compatible with conventional photolithography processes, the HBT can be made at low cost with high yields. A Si/Si1-xGex HBT also offers the ability to continuously adjust the bandgap of the heterojunction because Si and Ge are solid-soluble in each other to substantially any percentage.
FIG. 1 is a cross-sectional view illustrating an npn-type, Si1-xGex HBT, 100. A collector region 105 is formed within an upper portion of a crystalline silicon substrate layer 102 that is centrally disposed between device-isolating shallow trench isolators 104. The crystalline silicon substrate layer 102 is grown epitaxially and an n-type impurity, such as phosphorus or arsenic (P, As), is introduced into an upper portion of the crystalline silicon substrate layer 102 during epitaxial growth or by subsequent ion implantation. The n-type impurity forms an n-type doped region within the silicon crystal that performs as an n-type crystalline collector region 105 for the HBT.
An undoped Si1-xGex layer 120 is formed over the n-type crystalline collector region 105 by epitaxial growth of an admixture of a silicon-containing gas, such as silane (SiH4), and a germanium-containing gas, such as germane (GeH4). A p-type doped crystalline p+Si1-xGex layer 125 is epitaxially grown on the undoped Si1-xGex layer 120, to form an intrinsic base of the HBT over the n-type crystalline collector region 105. Another gas, such as borane (B2H6), containing the p-type impurity, boron (B), is added to silicon-containing and germanium-containing gases, for epitaxial growth of the p-type doped crystalline p+Si1-xGex layer 125. The p-type doped crystalline p+Si1-xGex layer 125, which forms the intrinsic base of the HBT, is electrically connected to an extrinsic base layer 128 of the HBT.
An undoped crystalline silicon cap 130 is epitaxially grown over the intrinsic base of the p-type doped crystalline p+Si1-xGex layer 125. Together, the undoped crystalline silicon cap 130, and the p-type doped and undoped crystalline Si1-xGex layers 125, 120, respectively, form a Si/Si1-xGex heterojunction.
Referring to FIG. 1, an emitter opening is formed above a central portion of the undoped silicon cap 130, through the extrinsic base layer 128 and an insulating layer 140. The emitter opening may be lined with insulating sidewalls 137. An n-type doped non-crystalline polysilicon 135, which includes an n-type impurity, such as phosphorous or arsenic (P, As), is deposited using a silicon-containing gas and a phosphorus-containing or arsenic-containing gas within the emitter opening and over the undoped silicon cap 130. The structure is heated, which causes the phosphorus or arsenic (P, As) of the n-type doped non-crystalline polysilicon 135 to diffuse into the central portion of the undoped crystalline silicon cap 130, to form an n-type diffusion-doped crystalline emitter for the HBT. Subsequently, the n-type doped non-crystalline polysilicon 135 may be patterned, etched and thermally annealed to form an emitter lead electrode 135.
It should be noted that the emitter/base/collector junctions of the HBT of FIG. 1 are partitioned from one another, not by the boundaries of the Si/SiGe crystals, but by the concentration profiles of the doping impurities.
There remains a need to improve high-speed operation of heterojunction bipolar transistors (HBTs).